Building a Custom Live CD with Linux-Live

A reader comment in last week's LWN, requesting a more detailed round-up of the available tools for building custom live CDs, inspired today's feature. Although there are more than a hundred bootable Linux, BSD and OpenSolaris-based distributions for seemingly every taste and purpose, it might still be useful, on occasion, to build one's own - customized to one's exact needs. The reasons are unimportant since they are likely to vary as much as the end result of any such undertaking. Instead, I'd like to concentrate on what the readers are probably most interested in - finding an answer about how much effort is required to build a custom Linux live CD and whether the end result is worth it.

I have never built a live CD before. For my first attempt I decided to go with the Slackware-based Linux-Live script by Tomas Matejicek, the author of the SLAX live CD. The reason? I expected Linux-Live to be about the simplest way to build a Linux live CD. I don't have any facts to back up this statement, but the increasing abundance of Slackware-based live CD projects, most of which use Linux-Live to build their products, together with the uncomplicated nature of Slackware itself, gave me confidence before embarking on this project. Incidentally, Linux-Live is released under the GNU General Public License.

Linux-Live is a script designed to build a bootable live CD from an existing Linux installation. A number of prerequisites must be fulfilled before the script can be executed; the most important among them is support for Unionfs and Squashfs modules in the Linux kernel. There are two ways to go about fulfilling this requirement: you can either download and compile two overlay filesystem modules, or download a pre-compiled kernel from the Linux-Live web site with all the necessary modules (as well as ALSA and proprietary MadWiFi drivers) already included. The latter option, however, will only work on a Slackware installation. The Unionfs and Squashfs requirements also mean that only kernel versions 2.6.9 or higher are supported.

After going through the documentation included in Linux-Live, I set out to create my live CD. First, I installed a minimal Slackware 10.1 system, with only the packages in a/, ap/ and n/ selected for installation. This was to reduce the possibility of a failure and also to minimize any time-wasting in case something goes wrong. Since Slackware still defaults to the 2.4 kernel series, I also downloaded the binary kernel 2.6.13.2 from Linux-Live.org and installed it with pkginstall. It is not necessary to reboot into the new kernel; as long as it is installed on the system and the correct kernel version is specified in the 'config' file of the Linux-Live script, it can be used. Then I download the Linux-Live script, decompressed and untarred it in the /tmp directory, updated the 'config' file with the new kernel version, and executed the rumme.sh script. After about six minutes of hard work, a 160 MB livecd.iso file was generated.

I burned it onto a CD-RW and rebooted the system. I held my breath, half-expecting the system to ignore the CD and just present me with the usual boot loader, but to my pleasant surprise, it was a SLAX logo that first appeared on the screen, indicating that the CD was indeed bootable. After I pressed 'Enter', the live CD went through the boot process - it took its time, I might add, since besides the normal boot procedure, the operating system also completed a hardware detection and configuration step. But eventually it stopped at a boot prompt, ready to accept a login by any of the user/password combination -- and with the same home directories -- as the Slackware system on the hard disk. Networking was also configured.

Encouraged by this success, I decided to try something more ambitious. I rebooted into my Slackware 10.1 installation, then added all software from the x/ and kde/ and l/ directories, configured X.Org, modified the /etc/inittab file to boot into a graphical login prompt, and repeated the process of generating the live CD. This time, the routine took much longer, around 20 minutes, and the resulting ISO image was 496 MB in size. Again, I burned it to a CD-RW disk and rebooted the system. Then I watched, with a considerable amount of amazement, as the CD went through a normal boot process before confidently starting the KDM login manager. Success!

I spent an afternoon re-creating Slackware live CDs in various configurations. While the process seemed to go smoothly most of the time, I noted a few mysterious "gotchas" on occasion. As an example, sometimes the original Slackware installation would no longer boot after running the ISO build script (the boot process would stop at Loading Linux...). There is also a documented problem with the fact that Slackware tries to test the status of the root partition by re-mounting it read-only, a test which fails on a live CD with Unionfs, resulting in a warning message and requiring user input. A minor, but annoying blemish. A few of my live CDs also failed to boot, for reasons unknown to me, with a "init Id 'x' respawning too fast" error. Probably the most serious issue with Linux-Live, however, is lack of documentation (apart from a few "readme" style files included with the script) explaining the process and providing hints for using the script on distributions other than Slackware. The project would also benefit from having a Wiki as well as user forums which adventurous live CD builders could use to search for answers and exchange experiences.

All things considered, my first attempt at building a custom live CD was a success. As I suspected before I embarked on the project, Linux-Live is a very simple and fast method for creating a live CD from a Slackware installation. Due to lack of time to research topics on compiling Unionfs and Squashfs as kernel modules, I haven't tried it on any other distribution, but the project's web site does give an impression that the script is fairly universal. However, Linux-Live badly needs better documentation and user interactive areas for those times when things don't go as expected.

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